Electric-Field-Controlled Nonvolatile Magnetization ... - ACS Publications

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Functional Inorganic Materials and Devices

Electric Field Controlled Non-volatile Magnetization Rotation and Magnetoresistance Effect in Co/Cu/Ni Spin Valves on Piezoelectric Substrates Wenbo Zhao, Weichuan Huang, Chuanchuan Liu, Chuangming Hou, Zhiwei Chen, Yuewei Yin, and Xiao-Guang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03761 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Electric Field Controlled Non-volatile Magnetization Rotation and Magnetoresistance Effect in Co/Cu/Ni Spin Valves on Piezoelectric Substrates Wenbo Zhao,† Weichuan Huang,† Chuanchuan Liu, † Chuangming Hou, † Zhiwei Chen,† Yuewei Yin,*,† and Xiaoguang Li*,†,‡ †

Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, and CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei 230026, China

‡

Collaborative Innovation Center of Advanced Microstructures, Nanjing, 210093, China

ABSTRACT: Electric field control of magnetism is a key issue for the future development of low-power spintronic devices. By utilizing the opposite strain responses

of

the

magnetic

anisotropies

Co/Cu/Ni/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3

in

Co

(PMN-PT)

and

Ni

films,

a

spin-valve/piezoelectric

heterostructure with ~7 nm Cu spacer layer was properly designed and fabricated. The purely electric field controlled non-volatile and reversible magnetization rotations in the Co free layer were achieved, while the magnetization of the Ni fixed layer was almost unchanged. Accordingly, not only the electroresistance but also the electric field tuned magnetoresistance effects were obtained, and more importantly at least six non-volatile magnetoresistance states in the strain tuned spin valve were achieved by setting the PMN-PT into different non-volatile piezo-strain states. These findings highlight potential strategies for designing electric field driven multi-state spintronic devices.

KEYWORDS: Multiferroic heterostructures; Piezo-strain effect; Non-volatile; Magnetization rotation; Electric field controlled spintronics. 1 / 24

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1. INTRODUCTION Non-volatile magnetization manipulation in multiferroic and magnetoelectric materials using pure electric fields instead of magnetic fields or large currents is highly desired in ultralow-power spintronic devices such as field sensors, magnetic random access memories, and spin logics.1-6 Recently, there have been a lot of efforts aiming at electric field control of magnetizations in multiferroic heterostructures through different mechanisms, including field effect,7-9 exchange coupling,10, exchange

bias,12

and

strain

effect.13-15

Among

various

mechanisms,

11

the

strain-mediated coupling in magnetostrictive/piezoelectric structures has been proved to be a very energy efficient way to achieve non-volatile magnetization manipulation.16-18 Most importantly, the strain effect can manipulate not only the amplitude but also the

orientation of the

magnetization in multiferroic

heterostructures,19-22 which is extremely promising for the design of spintronic devices.23, 24 For example, benefiting from the electrical controlled lattice strain in 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT)25-27 and BaTiO322 ferroelectric single crystals with large piezoelectric coefficients, non-volatile magnetization rotations in ferromagnetic (FM) films have been demonstrated in La0.6Sr0.4MnO3/PMN-PT,19 Co/PMN-PT,20, 21 and Ni/BaTiO322 etc. multiferroic heterostructures. On this basis, it has been predicted by theory that high densities and low power logic or memory functions can be realized in ‘straintronic’ devices by integrating strain effect in spintronic devices including spin valves or magnetic tunnel junctions,26 and several strain induced or assisted magnetization and magnetoresistance (MR) switchings have been experimentally confirmed.16, 28-30 For instance, the electric field manipulation of the coercive field of the free layer in a spin-valve grown on PbZr0.5Ti0.5O3 ferroelectric film has been demonstrated with the assistance of magnetic fields.30 2 / 24


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Purely electric field controlled magnetization rotations and the corresponding MR switchings were also achieved in the free layers of CoFeB-based magnetic tunnel junctions grown on PMN-PT substrates.16, 28, 29 However, the electric field induced magnetization and MR switchings in these ‘straintronic’ devices are volatile or require the assistance of magnetic fields, limiting their application potentials. The difficulty to integrate a complex spintronic device into electrically controlled multiferroic heterostructures has precluded the demonstration of non-volatile manipulation of magnetization, and thus purely electric field controlled non-volatile magnetization rotations and MR states have not been reported yet in any spin valves or magnetic tunnel junctions grown on piezoelectric materials. In this paper, we designed and fabricated a pure electric field controlled prototype

memory

based

on

a

Co/Cu/Ni/PMN-PT

spin-valve/piezoelectric

heterostructure. We found that the magnetic easy axis (MEA) of the top Co free layer went through a non-volatile 90° rotation, while the MEA of the bottom Ni fixed layer remained still, resulting in a variation of the MR effect in the spin valve. Interestingly, more non-volatile MR or electroresistance (ER) states could be obtained by setting the PMN-PT at different non-volatile piezo-strain states, and multi-state memories were obtained accordingly.

2. RESULTS AND DISCUSSION Figure 1 shows the magnetic properties of Co and Ni films in FM/PMN-PT(001) heterostructures, measured by a rotating sample magneto-optic Kerr effect (rot-MOKE) technique.31 With an electric field applied along pseudocubic [010] direction of the PMN-PT substrate, a non-volatile tensile strain in [010] direction is generated on the FM layer. The piezo-strain (ε) vs. electric field loops for PMN-PT substrate measured along [010] direction are shown in Figure 1b. The initial 3 / 24



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piezo-strain state ( Pi ) of PMN-PT substrate is defined as zero strain state (ε ( Pi ) = 0), and the piezo-strain with an applied electric field of +4 kV/cm (defined as Po+n4 state) is about ε ( Po+n4 ) = 0.13%. After the +4 kV/cm electric field is turned off, a non-volatile piezo-strain state ( Pr + 4 ) with ε ( Pr + 4 ) ~ 0.09% at zero electric field can be obtained. While after a -1.5 kV/cm electric field is applied and turned off, another non-volatile piezo-strain state ( Pr-1.5 ) with ε ( Pr-1.5 ) ~ 0.02% at zero electric field is achieved. Thus, two electric-field-controlled reversible and non-volatile piezo-strain states ( Pr-1.5 and Pr + 4 ) can be obtained at zero electric field by cycling the electric fields between +4 kV/cm and -1.5 kV/cm (see the red line in Figure 1b). Here, the non-volatile piezo-strain may be related to different aspects, such as the electric field induced rhombohedral to orthorhombic phase transformation,32 different ferroelectric domain configurations,33,

34

crystal miscut,35 and the existence of defects in the

near-surface region36 (see the supporting information Section 1 for the detailed discussion). In order to investigate the magnetic anisotropies of Co (Figures 1c-d) and Ni (Figures 1e-f) films manipulated via the in-situ piezo-strain controlled by electric fields, the detailed piezo-strain status dependencies of MEA (confirmed by the angle dependencies of the remnant magnetization ratio) were investigated using a rotating sample

magneto-optic

Kerr

effect

(rot-MOKE)

technique.31

Representative

normalized magnetic hysteresis loops at different strain statuses are shown in the supporting information Figure S2. Usually the uniaxial anisotropy in ferromagnetic metals or alloys could be initialized by applying an in-plane magnetic field higher than the saturation field during film deposition.37 For our samples, the saturation fields of Co and Ni at room temperature are about 50 Oe and 40 Oe, respectively (see 4 / 24


Page 5 of 24

the supporting information Figures S2b and d). Therefore, magnetic fields of 200 Oe in [100] or [010] directions, higher than the saturation fields of Co and Ni, were applied during the film growth, which was sufficient to locate the MEA of both Co and Ni along [100] (Figures 1c and e) or [010] (Figures 1d and f) directions, respectively. (a)

[001] polarized light

(b) 0.15

E // [010] P +4 r

[010]

0.10

ε (%)

[100]

FM

0.00

E

P +4 on

0.05

PMN-PT

P -r1.5 -4

Pi

(c)

P -r1.5 [100]

1.0

P +4 on

P +4 r (d) 1.0

MR/MS

[010]

Co/PMN-PT

[010]

0.5

1.0

[100]

1.0 (f)

Ni/PMN-PT

0.5 0.0

4

0.0

0.5

1.0

-2 0 2 E (kV/cm)

0.5

0.0

(e)

Pi

[100]

Co/PMN-PT

0.5

MR/MS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[100]

1.0

Ni/PMN-PT

0.5

[010]

0.0

0.5

0.5

1.0

1.0

[010]

Figure 1. (a) Schematic of the rot-MOKE measurement setup on the FM films grown on PMN-PT substrates. (b) Piezo-strain (ε) vs. electric field (E) loops for PMN-PT substrate measured along [010] direction at room temperature, from the initial state (black dashed line), between -4 kV/cm and +4 kV/cm (gray line), and between -1.5 kV/cm and +4 kV/cm (red line). Angular dependent remanent magnetization ratio at room temperature at different piezo-strain states for Co (c-d) and Ni (e-f) films, with initial MEAs located along [100] and [010] directions, respectively. For Co/PMN-PT heterostructure, when the initial MEA direction is located along [100], the MEA remains in [100] direction at Pr-1.5 state, and the MEA is 5 / 24



non-volatilely rotated 90° to [010] direction at Pr + 4 and Po+n4 states as shown in Figure 1c. This is because the positive magnetostriction effect of Co tends to align its MEA along the direction with relatively strong tensile strain which is [010] direction at Pr + 4 and Po+n4

states.20 On the other hand, if the as-grown MEA direction of Co

is along [010], the MEA directions are still along [010] direction with the tensile strains at Po+n4 , Pr + 4 , and Pr-1.5 states, as shown in Figure 1d. While for Ni/PMN-PT heterostructure, the negative magnetostriction effect of Ni tends to align its MEA perpendicular to the direction with relatively strong tensile strain.22 Thus, the MEA of Ni stays in the as-grown [100] MEA direction at the Pi , Pr-1.5 , Pr + 4 and Po+n4 states, as shown in Figure 1e. For the Ni film with the as-grown MEA direction

along [010] as shown in Figure 1f, the piezo-strain of Po+n4 state is large enough to rotate the MEA of Ni to [100] direction. While at Pr + 4 state without strong enough tensile strain, the MEA of Ni rotates back to [010] direction resulting in a volatile MEA rotation. Based on the different MEA rotation behaviors of Co and Ni films (Figures 1c and e), an electric-field-controlled Co/Cu/Ni spin valve could be designed with Co as a free layer and Ni as a fixed layer. Thus, Co(15 nm)/Cu/Ni(30 nm)/PMN-PT heterostructures with Cu thicknesses of 3 nm and 7 nm were constructed and the magnetic anisotropies of Co and Ni films were investigated by rot-MOKE at room temperature, respectively. The measurement setup is schematically shown in Figure 2a, with the electric field applied along [010] direction of the PMN-PT substrate, and the as-grown MEAs of the Co and Ni films are located along [100] direction as shown in the supporting information Figure S3. At Pr-1.5 state, no matter the Cu thickness is thin (3 nm, Figure 2b) or thick (7 nm, Figure 2c), the MEAs of Co and Ni films are 6 / 24


Page 6 of 24

Page 7 of 24

located along [100] direction, consistent with the results of Co and Ni single layers in Figures 1c and e. At Pr + 4 state, for the spin valve with 3 nm Cu layer the MEA of Ni film stays in [100] direction (Figure 2d), consistent with the result shown in Figure 1e; while the MEA of Co film also stays in [100] direction, different from the MEA rotation in single Co film in Figure 1c. This may be attributed to the strong coupling between the Co and Ni layers, which aligns the MEA of Co to be parallel to that of Ni.38 On the contrary, as shown in Figure 2e, for the spin valve with a thicker (7 nm) Cu layer at Pr + 4 state, the MEA of Ni stays in [100] but that of Co rotates to [010], consistent with the results of Co and Ni single layers in Figures 1c and e. The different responses of the spin valve with 7 nm Cu layer should be owning to the relatively weak magnetic coupling between Co and Ni layers. It implies that the strain effect dominates the magnetic anisotropies for the spin valve with 7 nm Cu layer. Therefore, the Co/Cu/Ni/PMN-PT heterostructure with 7 nm Cu could be adopted as an electric field controlled magnetoresistive prototype device. More importantly, the rotation of the MEA of the Co free layer is non-volatile and reversible. (b) 1.0

[001]

P -r1.5

[100]

[010] polarized lights [100]

MEA (c) Co Cu ~3 nm 1.0 Ni

0.5

MR/MS

(a)

Co

1.0

Cu

Ni

(d) 1.0

PMN-PT

E

0.5

P +4 r

[100]

[100]

Co Cu ~7 nm Ni

0.0

[010]

0.5 Co Ni 1.0

[001]

(e) Co Cu ~3 nm

1.0

Ni

0.5 [010]

0.0

MEA P -r1.5

0.5

[010]

0.0 0.5

MR/MS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.5

0.5

1.0

1.0

[100] P +4 r

[100]

[010] Co Cu ~7 nm Ni [010]

Figure 2. (a) Schematic of the rot-MOKE measurement setup on the spin-valve/PMN-PT heterostructure. Angular dependent remanent magnetization ratio 7 / 24



of Co and Ni layers in Co/Cu/Ni/PMN-PT heterostructures with 3 nm and 7 nm Cu layer, measured at room temperature at Pr-1.5 (b, c) and Pr + 4

(d, e) states

respectively. The insets schematically show the MEA directions in Co and Ni layers. [001]

(a)

(b) [010] SiO2

Co Cu

I+

Ti

~7 nm

Cu

I-

Ni

20 nm

Co

VV+

Ti/Au

[100]

(110)S

Ni

(200)S

PMN-PT (001) PMN-PT (001)

(c)

(d)

H//[100] 80 K 1.0

Pi

Pi Co

0.0

R (Ω Ω)

M/Ms

0.5

Ni

-0.5

0.00 348.4

0 H (Oe)

500

0.00 -0.02

348.3

-0.04 348.2

-0.06 0 H (Oe)

-500

0 H (Oe)

-0.10 1000

500

(f)

Pi

-500

-1000

1000

500

0.15

Pi

MR (%) MRAP (%)

-500

H//[010]

-1000

0.05

348.6

348.2

(e) 348.4

0.10

MRAP

-0.05

-1.0 -1000

H//[100] 348.8

MR (%)

E

R (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

0.10 0.05 0.00

1000

50

100 150 T (K)

200

Figure 3. (a) Schematic of the resistance measurement structure on a Co/Cu/Ni/PMN-PT heterostructure with a 7 nm Cu layer. (b) Cross sectional HRTEM image of the Co/Cu/Ni/PMN-PT heterostructure, the inset shows the SAED pattern. (c) Magnetic hysteresis loops for Co and Ni layers at Pi state, measured along [100] at 80 K. Magnetic field dependent resistance and MR at 80 K of an as-grown Co/Cu/Ni spin valve on PMN-PT with magnetic field along [100] (d) and [010] (e) directions. (f) Temperature dependent MRAP ratio measured with magnetic field along [100] direction. To demonstrate the functionality of this straintronic device, the electric field controlled MR properties of a cross-strip patterned Co(15 nm)/Cu(7 nm)/Ni(30 nm) spin valve grown on PMN-PT heterostructure were studied. Here, the MEAs of the as-grown Co and Ni films were aligned to be along the [100] direction of PMN-PT, and a current-perpendicular-to-plane (CPP) geometry was used, as shown in Figure 3a. 8 / 24


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Figure 3b shows the cross sectional high resolution transmission electron microscopy (HRTEM) image and the selected area electron diffraction (SAED) pattern of an as-grown heterostructure device. The interfaces between different layers are smooth. In the SAED pattern, the (200) and (110) reflections of the PMN-PT substrate are marked by white arrows, and the ring corresponds to the reflections of the polycrystalline Ni, Cu and Co films. The normalized magnetic hysteresis loops along [100] direction for the Co and Ni films in Pi state at 80 K are shown in Figure 3c. Here, the remanent strain states were induced at room temperature and then the samples were cooled to 80 K for magnetic and transport measurements. Consistent with the results at room temperature (see the supporting information Figure S3), the MEA directions of the Co and Ni films at Pi state stay in [100] direction at 80 K, and the coercive fields of Co and Ni films are about ±250 Oe and ±510 Oe respectively. It is noted that the resistance of our spin valve is higher than previous reported all-metal spin valves (10-8 to >10 Ω),39-44 which may be owning to the polycrystalline nature of our sample,45 the contribution from current-in-plane (CIP) resistance,44 and/or the possible weak oxidization of the 3d metals in the spin valve. Defining θ as the relative angle between the magnetization directions of Co and Ni layers in the Co/Cu/Ni spin-valve, the θ dependent CPP resistance R θ is given by46 Rθ = 1 − a × c o s 2 (θ / 2 ) . RAP

(1)

Here RAP is the CPP resistance of the spin-valve when the two FM layers are magnetically antiparallel (defined as AP state), and a is a constant. The CPP resistance of a spin valve increases as θ increases. Typical spin-valve MR effect was obtained when applying magnetic fields parallel to the MEAs of the Co and Ni layers as shown 9 / 24



in Figure 3d at 80 K. Sharp resistance switchings from magnetically parallel (P) state to AP state and from AP to P state are obtained when the magnetization of Co (at around ±240 Oe) and Ni (at around ±520 Oe) FM layers flip in magnetic fields, consistent with the coercive field values in Figure 3c. The MR effect is mainly related to the spin-valve effect, and the AMR contribution is small (see the supporting information Section 4). The minor R-H curve was also displayed in Figure 3d, indicating the non-volatility of P and AP states in zero magnetic field. For magnetic fields (along [010]) perpendicular to the MEAs of Co and Ni, the magnetization orientations of both layers rotate continuously, leading to the continuous changes of θ and R. Thus the R-H curves are smooth, as shown in Figure 3e. Figure 3f shows the temperature dependent MR ratio ( MR = ( R( H ) − R(0)) / R(0) ) at AP state (MRAP) of an as-grown spin valve, measured along [100] direction. As expected, the MRAP increases with decreasing temperature.47 Figures 4a-b show the normalized magnetic hysteresis loops at Pr-1.5 and Pr +4 states for the magnetic fields along [100] direction at 80 K, respectively. Similar to the results at Pi state (Figure 3c), the MEA directions of Co and Ni films at Pr-1.5 state stay in [100] direction at 80 K, while at Pr +4 state, the MEA of Co rotates to [010] direction and the MEA of Ni stays in [100] direction with its coercive field decreasing from 500 Oe at Pr-1.5 state to 320 Oe at Pr +4 state.48 At Pr-1.5 state, when the applied magnetic fields are parallel to the MEAs of Co and Ni, i.e. along [100] direction, the magnetic field dependent resistance loops show typical spin-valve MR curvature, as shown in Figure 4c. Sharp resistance switchings are obtained when the magnetization of Co (at around ±220 Oe) and Ni (at around ±500 Oe) FM layers flip in magnetic fields, consistent with the coercive field values (around ±230 Oe Oe for Co and ±500 Oe for Ni) in Figure 4a. While for the applied magnetic fields (along 10 / 24


Page 10 of 24

Page 11 of 24

[010] direction, Figure 4d) perpendicular to the MEAs of Co and Ni at Pr-1.5 state, the R-H curves are smooth, and the minor R-H loop exhibits volatile resistance changes. The MR behaviors observed at Pr-1.5 state (Figures 4c-d) are similar to those at Pi state (Figures 3d-e). On the other hand, at Pr +4 state, for magnetic fields (along [100] direction, Figure 4e) perpendicular to the MEA of Co and parallel to the MEA of Ni, the MR experiences a steep switching near the coercive field of Ni at approximately ±340 Oe, consistent with the magnetization behaviors of Co and Ni in Figure 4b. On the contrary, for magnetic fields (along [010] direction, Figure 4f) parallel to the MEA of Co and perpendicular to the MEA of Ni, the R-H curves at Pr +4 state show sharp resistance switchings around the coercive field of Co (approximately ±210 Oe). (b)

(a)

1.0

P -r1.5

0.5

Co

0.0

M/MS

Ni

-0.5

0.5 0.0

-0.5

-1.0

-1.0

H//[100] -500

0 H (Oe)

500

1000

H//[100]

(c)

(d)

0.10

0 500 H (Oe)

347.8

R (Ω Ω)

0.00

MR (%)

348.0

0.00

347.9

[010] mCo mNi

347.8

-0.05

1000

H//[010] P -r1.5

0.05

[100] -0.05

(f) 0.00

322.6

322.4

-0.05

(g)

III II

-7.25 -7.30

I

0.10

322.3 (h) -7.25

0.05

-7.30

0.05

-7.35

0.00

-7.40

-0.05

0.00

-7.40

-0.05

-7.45

-0.10 0 H (Oe)

500

0.00

-0.15

SR

-500

322.5 322.4

-7.35

-1000

0.05

-0.10

ER (%)

322.2

P +4 r

-0.05

V

IV

-1000

0.10

-0.10

-7.45

1000

MR (%)

322.6

R (Ω Ω)

322.7

MR (%)

0.05

P +4 r

∆MR (%)

R (Ω Ω)

-500

348.0

-1.5 348.2 P r

R (Ω Ω)

H//[100]

-1000

MR (%)

-1000

(e) 322.8

P +4 r

∆MR (%)

M/MS

1.0

ER (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-500

0 500 H (Oe)

1000

Figure 4. Magnetic hysteresis loops for Co and Ni layers at Pr-1.5 (a) and Pr +4 (b) states respectively, measured along [100] at 80 K. Magnetic field dependent resistance 11 / 24



Page 12 of 24

and MR of the spin valve along [100] and [010] directions, measured at 80 K at Pr-1.5 (c, d) and Pr +4 (e, f) states respectively. The olive and blue lines refer to magnetic fields sweeping from -1000 Oe to +1000 Oe and vice versa respectively, while the black square-spot-line lines refer to the minor loops. The insets schematically show the magnetization directions of Co and Ni layers. Magnetic field dependent ER and ∆MR of the spin valve along [100] (g) and [010] (h) directions. Comparing the resistances at different piezo-strain states, for example Figures 4c and e, it is found that the resistances of the spin valve at the magnetic P state in higher magnetic fields are different, which is due to a magnetization-rotation-irrelevant resistance change induced by pure strain variation,49 namely strain-resistance (SR) effect. In order to distinguish the contributions of the SR and the magnetization rotation on the resistance switchings, we calculated the resistance changes from Pr-1.5 to Pr +4 states controlled by electric fields as

ER ( H )= ( R + ( H ) − R - ( H ))/ R P- (0) .

(2)

Here R-(H) and R+(H) denote the resistance at Pr-1.5 and Pr +4 states respectively, and R P- (0 ) denotes the resistance at magnetic P state in zero magnetic field at Pr-1.5 state,

as shown in Figures 4g-h. In higher magnetic fields (Region I of Figure 4g and Region IV of Figure 4h), because the magnetizations of the Co and Ni layers are both parallel to the magnetic field direction, the MR effect could be eliminated, and the corresponding ER (approximately -7.35%) should represent the pure strain effect induced resistance switching. In principle, the strain-resistance SR ratio is basically magnetic field independent, as marked by the horizontal solid line in Figures 4g-h. Thus, the resistance switching induced by magnetization rotations can be calculated as

∆ M R ( H ) = ER ( H ) − SR .

12 / 24


(3)

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The ∆MR value in zero magnetic field (∆MR(0)) represents the resistance change caused by magnetization rotation controlled via purely electric fields without external magnetic fields. From the black square-spot-line in Figure 4g calculated from the minor R-H loops at Pr-1.5 and Pr +4 states, we can see that the ∆MR(0) values are approximately +0.03% and -0.03% for the θ changes from 0° to 90° and from 180° to 90°, respectively. This clearly indicates that the purely electric field induced non-volatile 90° magnetization rotation of Co layer causes approximately 0.03% resistance change in the spin valve, and accordingly the magnetization rotation could be read out by resistance measurement. In region II of Figure 4g, the ∆MR value in the green line is approximately -0.07%, denoting to the θ variation from 180° to 0°; while in regions Ⅲ (Figure 4g) and V (Figure 4h), the ∆MR is about +0.07%, corresponding to the θ switching from 0° to 180° . We could even set the PMN-PT into more different non-volatile piezo-strain states and achieve multiple electric-field-controlled MR effects. Figure 5a shows the

ε-E loops for PMN-PT substrate by cycling electric fields from +4 kV/cm to various electric fields (-1.5, -1.4, -1.3, -1.2, -1.1, and 0 kV/cm) along [010] direction, and six non-volatile piezo-strain states (marked as A to F) at zero electric field were obtained accordingly. With increasing tensile strain in [010] direction, the MEA of Co gradually rotates from [100] (the as-grown MEA) to [010] direction, as shown in Figure 5b, while the MEA of Ni layer remains along [100] but its coercive field decreases with increasing piezo-strain, as depicted in Figure 5c. The corresponding R-H curves measured at different non-volatile piezo-strain states for another spin valve sample with the same structure are shown in Figure 5e, and the calculated ER (the resistance changes from A state to different non-volatile piezo-strain states) and 13 / 24



∆MR in zero magnetic field at different non-volatile piezo-strain states are shown in Figure 5d. It is clearly shown that the amplitudes of both ER and ∆MR increase monotonously with increasing piezo-strain, and six different MR or ER states were obtained, corresponding to the six non-volatile piezo-strain states A to F, and demonstrating the potential of the straintronics in multistate memories. (a)

(b)

(e) 1.0 Co H // [100]

260.1 A B C D E F

0.0

-0.5

0.05 0.00 -2

-1.0

A 0 2 E (kV/cm)

4

(c)

-800 -400

0 400 H (Oe)

0 ER (%)

0.5 0.0 -0.5

F D E

0.010 0.005

A 0.000

-1.0 -1000 -500

0 500 1000 H (Oe)

248.3 D state

241.1 241.0

B C

-15

C state

248.4

0.015

-5 -10

254.4

0.020

H=0

A state

B state

254.5

800

(d) 1.0 Ni H // [100]

H // [100]

260.0

R (Ω Ω)

M/MS

εP (%)

0.5

F

0.10

∆ MR (%)

300 K

0.15

M/MS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

0.02 0.04 0.06 0.08 0.10 ε (%)

E state

234.5 234.4

F state 228.8 228.7 -1000 -500

0 500 1000 H (Oe)

Figure 5. (a) ε vs. E loops for PMN-PT substrate measured at room temperature, along [010] orientation, between +4 kV/cm and different negative electric fields. Normalized magnetic hysteresis loops along [100] for Co (b) and Ni (c) at 80 K respectively. d) ε dependent ER and ∆MR of the spin valve at zero magnetic field. (e) Magnetic field dependent resistance of the spin valve along [100] direction, measured at 80 K at various non-volatile piezo-strain states respectively. 3. CONCLUSIONS In

summary,

we

have

designed

and

fabricated

Co/Cu/Ni/PMN-PT

spin-valve/piezoelectric straintronic devices and obtained the electric field controlled non-volatile magnetization rotations in Co free layer and the corresponding MR changes in the spin valve. The non-volatile and reversible magnetization rotation and MR manipulation by purely electric fields without the assistance of magnetic fields show the significance of straintronics in the electric field controlled spintronic devices. 14 / 24


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In addition, at least six MR or ER states at different non-volatile piezo-strain states have been achieved, demonstrating its potential in multi-state memories.

4. EXPERIMENTAL SECTION Device Fabrication. The Co and Ni films, and Co/Cu/Ni multilayer films were deposited on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystal substrates (5 mm×5 mm×0.5 mm) using an ion beam sputtering technique at room temperature, and a 200 Oe magnetic field along [100] or [010] direction was adopted during the films preparation. The spin valve with a size of top Co electrode of 10 µm×10 µm was patterned into a cross-strip geometry by a three-step UV photolithography and Ar ion milling process, while the size of the bottom Ni electrode is 100 µm×500 µm. The top and bottom leads were separated by a 100 nm thick SiO2 film with resistance about 1010 Ω. Here, four different spin-valve samples were patterned on one PMN-PT (001) substrate, and then the PMN-PT was cut into 1.2 mm×1.2 mm×0.5 mm piece with one spin-valve. The side polarization electrodes for PMN-PT polarization were then grown with a separation of 1.2 mm (as shown in Figure 3a), and the maximum applied voltage was 480 V (4 kV/cm). Characterization. The transmission electron microscopy (TEM) image, and the selected area electron diffraction (SAED) pattern of the as-grown spin-valve/PMN-PT heterostructure cross section were characterized with a high resolution transmission electron microscope (HRTEM, JEOL JEM-ARM200F microscope operating at 200 keV, equipped with a spherical aberration corrector on the condenser lens system). The in-plane piezo-strain was monitored by a Radiant Technologies Precision Premier II tester with a laser interferometric vibrometer. The temperature-dependent dielectric constant of PMN-PT crystal was measured at 1 kHz using an LCR meter (Agilent 4294A). Rot-MOKE measurements were performed with a NanoMOKE II system 15 / 24



(Durham Magneto Optics Ltd., UK). The electrical transport properties were examined in a physical property measurement system (PPMS, EverCool II, Quantum Design). The out-of-plane resistance of the spin valve was measured using a four-point probe method with a bias current of 100 µA, and the voltage drop on the spin-valve ranged from about 20 mV to 40 mV, depending on the resistance of the spin valve.

ASSOCIATED CONTENTS Supporting Information. The discussions of the possible origins for the non-volatile piezo-strain states in PMN-PT, normalized magnetic hysteresis loops of Co and Ni films, as-grown MEAs of the Co and Ni films in a spin-valve, and the CPP resistance of Co and Ni films are included. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (X.G.L.) *Email: [email protected] (Y.W.Y.) ORCID Xiaoguang Li: 0000-0003-4016-4483 Yuewei Yin: 0000-0003-0965-4951 Note The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China, and 16 / 24


Page 16 of 24

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National Key Research and Development Program of China (2016YFA0300103 and 2015CB921201), and this work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.

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TOC graphic [001]

∆MR +4 kV/cm [010] V-

Ti/Au [100]

Co

I+

Cu

-1.5 kV/cm I-

Ni

MEACo

V+

SiO2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MEANi

PMN-PT (001)

E 0.02

0.04

0.06

0.08

0.10 ε (%)

24 / 24


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Electric-Field-Controlled Nonvolatile Magnetization ... - ACS Publications

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